Response time is a product of server performance. Therefore, those tools that track your server activities are the tools that measure response times. Your ability to access and interpret response-time data will depend on your server model, i.e., where your site resides.
Understand What Your Web Host Can Tell You If you use a hosting service, response-time measurements should be part of the data available to you. In fact, aggregate response-time reports for hosting services - whether issued by the service or an independent reviewer - are a great way to evaluate a potential host. Make sure your host or host candidate compares favorably in side-by-side response-time reviews and that they can make response-time figures for your own site available to you on demand.
Learn about Tools You Can Use to Do It Yourself Unless you're pretty adept at scripting languages, you'll want to leave this one to the hosts and the third-party services. If you're up for a technical challenge, you can follow the instructions given in Web Performance Measuring Tools and Services, a respected book on the subject, for creating a response-time monitoring application.
Find Out How Third-Party Services Can Help When it comes to measuring response times, a monitoring service can be a savvy investment. Most such services can present complex server-response data to you in an easy-to-understand (usually graphic) interface. What's more, you can often measure your site against others in your industry to see how your response times compare. For example, look at how WebPartner's SecretShopper service presents weekly response-time data in this graph.
One company's average response times (left bars), compared to industry group averages (right bars) for one week. Response time is the time it takes for the server hosting your site to acknowledge the browser's request to view the site. (Source: WebPartner)